A262769 Integers k such that the concatenation of 2^k and k is prime.

3, 23, 63, 261, 281, 291, 4689, 11231, 12519

Offset

1,1

Comments

First three primes: 83, 838860823, 922337203685477580863.

a(10) > 120000. - Giovanni Resta, Apr 08 2016

Links

Table of n, a(n) for n=1..9.

Example

For k = 23 we have 2^23 and 23 equal to 8388608 and 23, respectively, and 838860823 is a prime number.

Mathematica

Select[Range@ 5000, PrimeQ[2^# * 10^IntegerLength[#] + #] &] (* Giovanni Resta, Apr 08 2016 *)

Prog

(Python)
from sympy import isprime
def afind(limit):
  k, twok = 0, 1
  while k <= limit:
    if isprime(int(str(twok) + str(k))): print(k, end = ", ")
    k, twok = k+1, twok*2
afind(2000) # Michael S. Branicky, Mar 23 2021
(PARI) isok(k) = isprime(eval(Str(2^k, k))); \\ Michel Marcus, Mar 23 2021

Crossrefs

Cf. A000079.

Sequence in context: A216418 A254626 A362540 * A298393 A299511 A299311

Adjacent sequences: A262766 A262767 A262768 * A262770 A262771 A262772

Keyword

nonn,base,more

Author

Emre APARI, Mar 24 2016

Extensions

a(8)-a(9) from Giovanni Resta, Apr 08 2016

Status

approved